FILE - The sun sets behind the Kukulkan Pyramid in the ancient Mayan city of Chichen Itza in Mexico on Thursday, Dec. 20, 2012. A study published online Thursday, Aug. 1, 2013 by the journal Science says as the world gets warmer, people are more prone to get hot under the collar. Scientists found that aggressive acts like violent crimes and wars become more likely with each added degree. Solomon Hsiang, author of the study, pointed to the collapse of the Mayan civilization that coincided with periods of historic drought about 1200 years ago. (AP Photo/Israel Leal)